There has conventionally been known a control device configured to control an upper limit of charge voltage (upper limit voltage) upon charge of a lithium ion battery (see Patent Document 1). The control device increases the upper limit voltage in accordance with capacity degradation of the lithium ion battery to be charged. The lithium ion battery can thus have substantially constant battery capacity available even if the lithium ion battery has capacity degradation.
Increase in upper limit voltage upon charge of the lithium ion battery may, however, lead to serious degradation in power and battery capacity of the lithium ion battery. Increase in upper limit voltage upon charge by the control device or the like along with degradation of the lithium ion battery may cause rapid degradation of the lithium ion battery.